This invention relates to star polymers and, more particularly, to the carbocationic polymerization of functionalizable copolymer arms emanating from a calixarene core to form the star polymers. Specifically, this invention relates to the synthesis of star polymers by living carbocationic copolymerization of statistical polymers comprising monomers of isobutylene (IB) and one or more other monomers selected from the group consisting of styrene, isoprene and carbocationically copolymerizable derivatives thereof. The star polymers are particularly suitable for use in coatings when reacted with crosslinking agents such as isocyanates.
The synthesis of various multi-arm radial or star polymers has grown in both practical and theoretical interest in a variety of industries. More particularly, the synthesis of well-defined star polymers having a readily determinable, definite number of arms have become increasingly more important over the years. Such star polymers are useful as, inter alia, surfactants, lubricants, rheology modifiers, viscosity modifiers, control agents, coatings and sealants.
There has been a growing interest in star polymers comprising multiple polyisobutylene (PIB) arms. For example, Kennedy et al. U.S. Pat. No. 5,395,885 describes the synthesis of star polymers having multiple PIB arms and polydivinylbenzene (PDVB) cores using cationic, xe2x80x9carm-firstxe2x80x9d, synthesis techniques. Because the structure of polyisobutylene is readily characterized and contains no unsaturation, these PIB-based stars are believed to be useful for a variety of applications such as motor oil additives and viscosity index improvers. The PDVB cores, however, were not xe2x80x9cwell definedxe2x80x9d, meaning that the core of the star polymer, e.g., PDVB, was an uncontrolled, crosslinked, gel-like structure having unsaturation sites in the core.
Consequently, further studies done by Kennedy provided for multiple PIB arms radiating from xe2x80x9cwell-definedxe2x80x9d cores which were built of readily characterizable, soluble molecules which are precursors to the core. As a result, the structure of the resultant star polymers having well-defined cores were able to be controlled. Included within this group of xe2x80x9cwell-definedxe2x80x9d cores was calixarenes. Kennedy et al. U.S. Pat. No. 5,844,056 describes the synthesis and characterization of star polymers having multiple, well-defined PIB arms emanating from a calixarene core. The synthesis was accomplished by the use of multifunctional calixarene derivative initiators which, in conjunction with certain Friedel-Crafts acids that acted like co-initiators, induced the living (carbocationic) polymerization of isobutylene.
Other Kennedy patents resulted from similar (yet different) synthesis techniques which resulted in star polymers of multiple PIB arms emanating from different well-defined cores. An example of these different cores included polysiloxane cores.
Once these star polymers were produced, other applications for star polymers were revealed. In at least one instance, production of novel thermoplastic elastomers (TPEs) became of interest. Consequently, star-polymers having poly(isobutylene-block-styrene) arms emanating from a calixarene core were synthesized. These star polymers became the subject of Kennedy U.S. Pat. No. 5,804,664. While these star polymers were suitable for their intented use, other applications and physical characteristics have now become of interest as discussed hereinbelow.
It is well known that living and quasi living polymerizations proceed in the absence of chain transfer and termination. By extending this definition, it is expected that living copolymerizations can occur in the absence of these chain-breaking reactions. In view of the inherent complexity of copolymerizations, however, it is far more difficult to achieve living copolymerizations than living homopolymerizations of a single monomer. This difficulty arises because copolymerizations involve at least two monomers whose reactivities are necessarily different, and because the complexity of the mechanisms of chain-breaking reactions escalates in the presence of two monomers.
Relatively few attempts have even been made at living carbocationic copolymerization. Orszagh et al. J. Phys. Org. Chem., 8, 258 (1995) have investigated the living carbocationic copolymerization of the isobutylene/p-methyl styrene system. As with homopolymerizations, these workers showed the absence of chain transfer to monomer in copolymerization by linearly ascending Mn versus conversion plots and corresponding horizontal N (number of polymer molecules) versus conversion plots. Further, the absence of termination was demonstrated by linear first order in monomer rate plots.
Some 35 years ago, Okamura et al., J. Polym. Sci. Polym. Chem. Ed., 3, 2455 (1965), investigated statistical IB/St copolymerizations mediated by TiCl4 and found that this system exhibited an azeotropic composition at 21/79 mol/mol IB/St. The effect of conversions on copolymerization compositions and Mn however, was not studied. Moreover, there has never been any suggestion for the production of such copolymers for use in star polymer synthesis.
To that end, the synthesis of star polymers with a range of glass transition temperatures (Tgs) has continued to be a significant challenge to polymer scientists and engineers. The production of a star polymer having copolymer arms whose Tg can be controlled between the Tgs of the polymers from which the copolymer arms are produced, and whose termini are fitted with crosslinkable endgroups, e.g., allyl, hydroxyl, etc., are of particular interest. Preferably, the production of a star polymer having multiple statistical poly(isobutylene-co-styrene) arms whose Tg can be xe2x80x9ctunedxe2x80x9d between about xe2x88x9273xc2x0 C. to about +100xc2x0 C. i.e., between the Tgs of polyisobutylene and polystyrene, respectively, are desired. Control of the length (i.e., number average molecular weight, Mn) of the copolymer arms is also desired.
It is therefore, an aspect of the present invention to provide a star polymer having multiple, statistical copolymer arms emanating from a calixarene core.
It is another aspect of the present invention to provide a star polymer, as above, wherein the Tg of the copolymer arms can be xe2x80x9ctunedxe2x80x9d or controlled between the Tgs of the polymers from which the copolymer was produced.
It is yet another aspect of the present invention to provide a star polymer, as above, wherein the termini of the copolymer arms are fitted with crosslinkable endgroups.
It is still another aspect of the present invention to provide a star polymer, as above, wherein the number average molecular weight of the copolymer arms can be controlled.
It is a further aspect of the present invention to provide a coating comprising a crosslinking agent and the star polymer above.
At least one or more of the foregoing aspects, together with the advantages thereof over the known art relating to star polymers and coatings, which shall become apparent from the specification that follows, are accomplished by the invention as hereinafter described and claimed.
In general the present invention provides a star polymer comprising a calix[n]arene core where n=4 to 16; and N number of arms of a statistical copolymer comprising monomers of isobutylene and a different, carbocationically copolymerizable monomer selected from the group consisting of styrene and carbocationically copolymerizable derivatives thereof, and isoprene and carbocationically copolymerizable derivatives thereof, connected to the calix[n]arene core, wherein N=n.
The present invention further provides a coating comprising the reaction product of at least one crosslinking agent and a star polymer containing a calixarene core and a plurality of poly(isobutylene-co-styrene) statistical copolymer arms, each arm having a functional end group that can be crosslinked by the at least one crosslinking agent, such that the star polymer is bound to itself or another star polymer.